About the Prefectura Naval Argentina

The PNA (Prefectura Naval Argentina (Argentine Naval Prefecture) is a service of the Argentine Interior Ministry charged with protecting the
country's rivers and maritime territory. It therefore fulfills the functions of other countries' coast guards, and furthermore acts as a
gendarmerie force policing navigable rivers.Esteban Rivera, 02 Oct 2011

Origin and description of the flag

The Prefectura Naval Argentina (PNA) is the Argentine maritime
law enforcement agency. It grew out of the port captaincies established
by the Spanish beginning in 1756 and was established under its current
name in 1896.

My translation from an unofficial Spanish
language website on the PNA:

The first known distinguishing pennant of the institution was included
in the approval of modifications to the signal code by the decree of August
1, 1903, signed by the President of the Republic, General Julio A. Roca, and
the Minister of the Navy, Admiral Onofre Betbeder. It consisted of a blue
flag with the initials "PM" in white in the center.

In 1924, when that year’s edition of the aforementioned code was approved,
the distinguishing flag was modified in favor of that currently used, that
is to say a white flag surrounded by a blue border, enclosing two crossed
anchors of the same color in the center. It was used only in the bows of
vessels of what was then the General Maritime Prefecture until April 25,
1941, when it began to be hoisted on the masts of all the organs and
dependencies of the institution as well.

In 1934 the command flags of the top authorities of the institution were
established, consisting of the previously described flag with three red
five-pointed stars for the National Naval Prefect, two for the National
Naval Subprefect, and one star for prefects of zones.

Unfortunately, there’s no indication of how these stars are arranged
on the flag.

The PNA website
says the following about the badge and flag used by the service:

The current PNA badge — two crossed anchors — had already been
used for letter headings at the times of the old Captainships of the Port in Buenos Aires by
1834. On October 6, 1893, it was included — by Decree — as a part of the
escutcheon on the front side of the top rank officers’ caps. In 1924, it was
incorporated into the PNA insignia, which consisted of a white,
blue-bordered flag and two centered anchors in the same colour.

The flag was in use on the official correspondence of the Capitanía de Puerto de Buenos Aires (ca. 1834). It was officially established by
Decree on October 6, 1893, to be worn by its members (cadets) on their hats. In 1924 it became the guidon of the PNA.
Source: http://www.prefecturanaval.gov.ar/web/es/html/inst_historia.php
Esteban Rivera, 2 Oct 2011